Autonomic neurons in the spinal cord of the rhesus monkey: A correlation of the findings of cytoarchitectonics and sympathectomy with fiber degeneration following dorsal rhizotomy

Abstract

Cytoarchitectonic study of cell groups in the zona intermedia of the spinal cord led to the identification of the following nuclei in the rhesus monkey: n. intermediolateralis thoracolumbalis pars principalis et funicularis (ILp, ILf); n. intercalatus spinalis (IC); n. intercalatus pars paraependymalis (ICpe); n. intermediomedialis (IM); n. intermediolateralis sacralis pars principalis et funicularis (ILSp, ILSf); n. intercalatus disseminata (ICd); and n. torsade. Unilateral thoracic or abdominal sympathectomy, or both, performed in a second group of macaques, resulted in massive numbers of chromatolytic preganglionic neurons in IC, ILp and ILf. In the third group of macaques, dorsal rhizotomy followed sympathectomy. Fiber degeneration in these cases was stained with the Nauta and Fink‐Heimer methods. Despite variations in survivaltime and staining methods, no evidence was found for dorsal root terminal connections with the somata or proximal dendritic stems of chromatolytic sympathetic or parasympathetic visceral motor neurons (IC, ILp, ILf, ILSp, ILSf) although the same cases contained terminal fibers in n. marginalis of Waldeyer, substantia gelatinosa of Rolando, n. proprius cornus dorsalis (Pd), Clarke's column, n. intermediomedialis (IM) and among axial and appendicular somatic motor neurons. The present anatomical data suggest that, within the spinal cord, visceral reflex pathways are polysynaptic. Neurons in two or more spinal cell groups, IM, Pd and Cb (central basilar area of the dorsal horn) perhaps act as intermediary cells connecting dorsal root visceral afferent fibers with visceral motor (preganglionic) neurons. The visceral reflex pathway thus described includes a four neuron chain: dorsal root cell; IM, or Pd neuron, or both; preganglionic neuron; and the postganglionic neuron of the sympathetic system in the paravertebral or collateral ganglia, or the parasympathetic postganglionic neuron lying more distally. It was also found that many preganglionic neurons of the ILp, ILf and IC cell groups are as large as somatic motor neurons. Furthermore, the impression gained in this study is that visceral motor neurons appear in far greater numbers than somatic motor neurons.